1. Field Of The Invention
This invention relates generally to photoinitiators used in cationic polymerization, and specifically to the synthesis and use of a new class of diaryliodonium salt photo and thermal initiators with aryl-substituted alkoxy groups bearing therewith hydroxy groups.
2. Discussion Of The Art
In the past several years, photoinitiated cationic polymerization has received considerable attention as a rapid, energy efficient and pollution-free method for the cure of epoxy monomers. It has been discovered that certain onium salts, namely diaryliodonium (I) and triarylsulfonium (II) salts, rapidly and efficiently photoinitiate the polymerization of practically all types of cationically polymerizable monomers: ##STR1##
Because the above classes of compounds could be synthesized with relative ease, together with high quantum yields of photolysis and exceptional thermal stability in the presence of monomers, they became the first truly practical photoinitiators for cationic polymerization. These facts and others are pointed out clearly and succinctly in a paper prepared by the instant inventor and colleagues Conlon, Olson and Webb, entitled: "The Effects Of Polyols As Chain Transfer Agents And Flexiblizers In Photoinitiated Cationic Polymerization", Journal of Radiation Curing, 3-9, October 1986. Within the industry, diaryliodonium and triarylsulfonium salts continue to be the principal photoinitiators for cationic ultraviolet (UV) curing. Diaryliodonium salts having the structure shown in (I) have been described in the patent literature as efficient photoinitiators for cationic polymerization, notably in U.S. Pat. No. 4,683,317 issued in 1987 to the instant inventor, J. Crivello, and colleague J.L. Lee. Therein, the subject salts result from the condensation of aryliodosotosylates and aryl ketones. The salts are used as photoinitiators to effect deep section UV cures in (deep section) photopolymerizable organic materials used for the encapsulation of electronic components.
As pointed out in the Journal of Radiation Curing article, there are two major advantages to UV curing using photoinitiated cationic polymerization. The curing systems require no blanketing by inert gas because they display no inhibition by oxygen; and, many of the photoinitiated cationic polymerizations undergo substantial post cure after initial irradition. The postcuring property, which can be accelerated by heating, may readily suffice in applications where an immediate cure of the coating, having been inadequate, is not absolutely essential.
Notwithstanding, the existing postcure effect, it has been determined that when multifunctional epoxy monomers are UV cured, using photoinitiated cationic polymerization, the cure rate passes through a maximum, and then falls off rapidly. This factor is revealed also in the aforementioned article. The same effect is observed when multifunctional vinyl monomers are polymerized using free radical photoinitiators. In such network forming systems, polymerization proceeds rapidly until gelation occurs. At this point, polymerization markedly slows, but can still occur as monomers diffuse to the fixed propagating sites within the swollen gel. As the gelation temperature of the network reaches the region of the temperature at which UV curing is taking place, vitrification sets in, monomer diffusion is impeded, and polymerization virtually ceases. Delaying the onset of gelation so that it closely approaches the point of vitrification assures an overall high polymerization rate, thus obviating existence of sizable unreacted (but polymerizable) groups within the UV-cured coating. The instant inventor and his colleagues determined that, in photoinitiated cationic epoxy systems, such results can be achieved with the use of chain transfer agents, specifically hydroxyl-containing compounds such as water and alcohols.
In U.S. Pat. No. 4,090,936, issued to Barton in May 1978 and entitled "PHOTO HARDENABLE COMPOSITIONS", a class of diaryliodonium salts is disclosed in which aryl groups have been defined as composed of aromatic groups or arylalkyl groups containing from 6 up to, but not more than, 20 carbon atoms. Salts having more carbon atoms were specifically excluded from this disclosure. In another patent recently issued to the instant inventor, U.S. Pat. No. 4,882,201, diaryliodonium salts possessing alkoxy groups of differing lengths were described as non-toxic photoinitiators; however, no disclosure is made therein of similar diaryliodonium salts containing other functional groups, particularly with long carbon chains. Most notably, there is a complete paucity in either the patent or chemical literature of descriptions or disclosures of diaryliodonium salts possessing hydroxyl-containing alkoxy groups attached to the aryl moieties; such salts have now been made with high yields and are hereinafter described. Likewise, the aforementioned related application teaches the method for obtaining significant (quantitative) yields of soluble diaryliodonium salts having alkoxy groups of more than twenty carbon atoms attached to an iodine atom.